Question

each bus in vivians city has a unique id number. she wants to know how many years a typical bus has been in service.
which method can vivian use to choose a random sample of city buses?
choose all of the buses that have been in service for more than 20 months.
write the id numbers of the 20 oldest buses on cards. then, choose five cards without looking.
list the buses in order from newest to oldest. then, choose the first 20 buses on the list.
use a computer to list the id numbers in a scrambled order. then, choose the first 20 buses on the list.

Explanation:

A random sample requires every bus to have an equal chance of being selected. The first three options introduce bias (filtering by service length, selecting only old buses, selecting ordered new/old buses). The fourth method scrambles IDs randomly, giving each bus an equal chance to be in the first 20.

Answer:

Use a computer to list the ID numbers in a scrambled order. Then, choose the first 20 buses on the list.